1. Field of the Invention
This invention relates to a particulate vinyl polymer and also to a water-base coating composition using the particulate vinyl polymer. More specifically, this invention is concerned with a particulate vinyl polymer excellent in workability such as the solubility in an aqueous medium and also with a water-base coating composition which can form coating films excellent in waterproofness, adhesion and the like and which also has excellent pigment dispersibility and is hence suited for paints, inks and the like.
2. Description of the Related Art
From the environmental viewpoint, there is an ever-increasing demand for water-base coating compositions in recent years, leading to the development of a wide variety of products.
In an emulsion, dispersion or the like of a polymer, the polymer is contained in a form distributed in water. The emulsion, dispersion or the like therefore does not exhibit any substantial increase in viscosity even when the molecular weight of the polymer becomes higher. This makes it possible to use a polymer of high molecular weight, leading to the advantage that a water-base coating composition capable of forming a coating film of excellent physical properties can be formulated. Numerous water-base coating compositions have hence been developed to date.
Whenever dispersion of a pigment at a high concentration is needed, however, it is indispensable to rely upon the action that an associated polymer surrounds the pigment to prevent its re-agglomeration. In an emulsion, dispersion or the like in which a polymer is contained in a form dispersed in water, it has however been difficult to fully prevent such re-agglomeration of a pigment, thereby failing to obtain sufficient pigment dispersibility. To surround the pigment for the prevention of its re-agglomeration, it is needed that the polymer must be contained in a form dissolved in water.. To this end, the development of water-soluble polymers has been proceeded with in various ways.
Typical examples of such water-soluble polymers include polyvinyl alcohols and sodium salts of polyacrylic acids. These water-soluble polymers are however accompanied by many practical inconveniences, because even after their formation into coating films, they are prone to dissolution in water and the coating films are thus re-dissolved and damaged by a slightest adverse cause such as exposure to water droplets.
To overcome the above inconveniences, a technique was developed. According to this technique, a polymer containing acidic groups such as sulfonic or carboxylic groups is poured into an aqueous medium containing a volatile base so that the polymer is neutralized into a salt and is hence rendered soluble in water. In the course of drying subsequent to coating, the volatile base is allowed to evaporate so that the polymer becomes insoluble. Water-base coating compositions based on this technique have been put on the market in recent years.
The above water-base coating compositions are however accompanied by the drawback that the dissolution of a polymer in an aqueous medium containing a volatile base takes time. It has accordingly been attempted to enhance the solubility of a polymer by increasing its acid value or comminuting it into fine particles. A coating film which has been formed from a polymer with an increased acid value however involves the problem that it has low waterproofness and cannot avoid deteriorations such as whitening. A comminuted, that is, fine particulate polymer, on the other hand, is accompanied by the problem that it tends to scatter around as dust upon handling and hence reduces workability.
Illustrative polymer particles having dust-free property include polymer powder such as those disclosed in Japanese Patent Laid-Open No. 242807/1990. They can be obtained by salting out fine particles, which have been produced by emulsion polymerization, and then collecting them by filtration. Such polymer powder however has irregular particle shapes and hence are poor in free-flowing property. When a worker attempts to take the polymer powder out of a bag to charge them into an aqueous medium, for example, the polymer powder does not flow out of the bag at a uniform rate, resulting in irregular charging into the aqueous medium. Upon dissolution of the polymer powder, polymer aggregates may be formed, resulting in the problems that the polymer powder may not be dissolved well and the workability is lowered.
It has also been attempted to blend a dispersion, emulsion or the like of a polymer having good waterproofness in an aqueous polymer solution. When dispersion of a pigment at a high concentration is needed, however, it is difficult to achieve both pigment dispersibility and waterproofness while holding a good balance therebetween.